Stellar Multiplicity Meets Stellar Evolution and Metallicity: The APOGEE View

We use the multi-epoch radial velocities acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to perform a large-scale statistical study of stellar multiplicity for field stars in the Milky Way, spanning the evolutionary phases between the main sequence (MS) and the red clump. We show that the distribution of maximum radial velocity shifts (ΔRV_max) for APOGEE targets is a strong function of log g, with MS stars showing ΔRV_max as high as ∼300 {km} {{{s}}}^-1, and steadily dropping down to ∼30 {km} {{{s}}}^-1 for log g ∼ 0, as stars climb up the red giant branch (RGB). Red clump stars show a distribution of ΔRV_max values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high ΔRV_max systems in the RGB is consistent with a lognormal period distribution in the MS and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe overflow during H-shell burning. The ΔRV_max distributions also show that the multiplicity characteristics of field stars are metallicity-dependent, with metal-poor ([Fe/H] ≲ -0.5) stars having a multiplicity fraction a factor of 2-3 higher than metal-rich ([Fe/H] ≳ 0.0) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.